The present invention relates to the removal of obstructions which develop in thimbles installed in nuclear power reactors, and relates more particularly to the safe removal of such obstructions while the reactor continues in normal operation.
During the operation of a nuclear power reactor, reactions within the core generate a neutron flux the spatial distribution of which must be mapped in order to insure safe and efficient operation of the reactor. For this purpose, reactors are provided with an array of tubes, termed thimbles, which extend into the reactor core and which are closed at their lower ends so that their interiors are isolated from the pressure existing within the reactor and can thus be maintained at normal atmospheric pressure. These thimbles define paths for flux detectors which can be advanced along the length of each thimble in order to produce neutron flux measurements at various points along the length of each thimble.
During reactor operation, it frequently occurs that obstructions develop in these thimbles due, for example, to the accumulation of lubricating material at points where the thimbles are provided with bends.
While complete removal of the material forming such obstructions requires reactor shutdown, there is a demand in the industry for a procedure which can at least temporarily clear an obstruction, without requiring reactor shutdown, so that the requisite number of thimbles can continue to be used for the flux mapping operation.
A known procedure for achieving this result, which will be described below, has been found to impose stresses on the fitting which secures the thimble to the reactor and has on occasion weakened the fitting to such an extent that the high pressure at the interior of the reactor has caused the thimble to be ejected from the pressure vessel, with an accompanying escape of radioactive coolant in the form of steam and/or water.